This tip explains what's needed.
https://discussions.apple.com/docs/DOC-2455
When getting Snow Leopard:
- Make sure the Mac is at least a CoreDuo or CoreSolo Intel. The G5 will not take it.
- Core2Duo iMacs can take 10.7 after installing 10.6.6 combo update on top of the 10.6.3 retail which can you can obtain from some sources on eBay and Amazon. Make sure they are ranked well.
- Mid-2007 and later iMacs can go up to 10.11 after installing 10.6.8 combo update on the aforementioned 10.6.3 retail.
Any iMac that shipped with Leopard can only be upgraded to 10.11.
If the data on the machine is not of import, you can use Target Disk Mode to clone over a relevant operating system that is compatible with the said Mac keeping in mind the license of the installed OS.
You can also use the migration assistant to import data onto a newer system installed on an external hard drive, provided there are no left over PowerPC applications.